fix(auth): merge configured and server-required OAuth scopes

[tsarlandie-oai]

Motivation and Context

Reactive OAuth discovery records scopes required by the server's
WWW-Authenticate challenge and validated protected-resource metadata. However,
AuthorizationSession::new bypasses the existing scope-selection policy whenever
AuthorizationRequest::with_scopes supplies explicit client scopes:

if request.scopes.is_empty() {
    request.scopes = auth_manager.select_scopes(None, &[]);
} else {
    auth_manager.add_offline_access_if_supported(&mut request.scopes);
}

As a result, a client configured with configured shared does not request
challenge or resource when the Bearer challenge and protected-resource
metadata require them. Duplicate configured scopes are also preserved, and
authorization can fail even though the SDK discovered the scopes necessary for
the operation.

What changed

• Route both configured and implicit scopes through the existing stable,
  deduplicating scope-selection policy.
• Seed that policy with explicitly requested scopes before accumulating
  previously requested scopes, Bearer challenge scopes, and protected-resource
  scopes.
• Keep authorization-server scopes_supported as a fallback only when no
  caller-requested or operationally required scopes exist.
• Preserve the existing offline_access policy and apply the same resulting
  scope set to preregistered clients, dynamic registration, and the authorization
  URL.
• Update public documentation to explain that configured scopes are combined
  with server-required scopes.

The public select_scopes API is unchanged. Discovery remains reactive,
consistent with #1052: this change adds no synthetic POST, ping, initialize
request, or additional discovery endpoint.

Regression coverage

Two end-to-end authorization tests cover preregistered and dynamically
registered clients with:

configured scopes:           configured shared configured
Bearer challenge scopes:     challenge shared
protected-resource scopes:   resource shared
authorization-server scopes: fallback-only offline_access

requested scopes:            configured shared challenge resource offline_access

The tests verify stable ordering, deduplication, exclusion of unrelated
authorization-server scopes, preserved offline_access, reactive GET-only
metadata discovery for preregistered clients, and identical scopes in the
dynamic-registration request and authorization URL.

Validation

• cargo test -p rmcp --features auth --lib transport::auth::tests — 173 OAuth
  tests passed.
• cargo test -p rmcp --features auth,client,transport-io — full package tests
  and doctests passed.
• cargo clippy -p rmcp --features auth,client,transport-io --lib -- -D warnings.
• rustfmt --check --edition 2024 crates/rmcp/src/transport/auth.rs.
• git diff --check.

Breaking Changes

None. Existing public APIs, registration priority, metadata fallback, and
reactive discovery behavior are preserved.